1. Field of the Invention
This invention relates to improvements in circuits for sensing load currents, and more particularly to improvements in load current sensing circuits which can be used to sense currents in multiple load applications, such as in polyphase DC motor systems or the like.
2. Description of the Prior Art
Information concerning current flowing in a load is often sought for various applications, for example, in current limiting, transconductance control, fault detection, and so forth. It is usually desirable that such information be developed without dissipating energy which would otherwise be delivered to the load. Thus, such information is often obtained by various non-interfering methods. For example, one way such information has been developed in the past is by sampling the magnetic field induced by the current flowing in selected conductors. This approach, however, is usually undesirably expensive for a reasonable precision. Another way has been used is to insert a resistance or other element in series with the load and sample the voltage developed across it. Although this approach is less expensive than the first, it requires the expenditure of some output energy in the sensing element.
Other approaches are also known, such as the use of a sense FET, or through current mirroring. One common approach is by inserting a resistor in series with the power supply (usually at the ground terminal) and sensing the voltage developed across it. This approach has been used in multiple load systems, such as multiple winding motor loads in which one or more windings are energized in series at any given time. An example of this approach is illustrated in FIG. 1. As shown, an FET switching circuit 10 switches voltage from V.sub.cc through selected windings of a motor 12 to ground. A current sensing resistor 13 is connected in series between the switching circuit 10 and ground. The voltage across the resistor 13 indicates the current flowing through the load, i.e., the selected windings of the motor 12. This approach, however, also has the disadvantage that it dissipates part of the available power to the motor.
One technique which has been employed in motor driver circuits uses a plurality of sense resistors, each in series with one or more of the driver transistors. One such circuit, for example, uses four transistors connected in two series paths, each series path including a sense resistor. The motor is connected between interconnecting nodes of each path, and the transistors are switched to provide current paths through the motor including alternatingly a first transistor from one path and a second transistor from the other. The voltage drops on each resistor are compared by a differential amplifier to generate an error signal to correct the bias on the driver transistors. However, such system requires separate resistors for each series path, and requires more complex comparing circuits if more than two current paths are employed.